1. Field of the Invention
The present invention relates generally to the field of signal processing, and more specifically to a method and apparatus for calibration of a signal processing system utilizing digital down converters.
2. Description of Related Art
Acoustic listening systems for underwater applications are well known in the art. For example, modern military submarines are equipped with arrays of acoustic sensors which provide sensitive underwater listening capabilities and even relative position information. Each sensor reacts to an incoming pressure wave by modulating an input signal and the outputs of all the sensors are processed to determine the sound and position information. These sensor arrays are generally either attached to the submarine hull or are towed behind the submarine. Ideally, the sensors would be attached to the submarine hull, but prior art acoustic sensors were simply too heavy for many submarine applications. Recent advances in acoustic sensor array technology, however, have produced arrays which are light enough to be mounted on a submarine hull and which still provide very sensitive signal response. This weight reduction has also allowed the number of sensors to be increased.
Another application for underwater acoustic sensors is in the geological survey industry, specifically, for underwater oil exploration. Vast arrays of sensors may be placed on the ocean bottom in the vicinity of known oil reserves. A surface ship then initiates an acoustic pressure wave (i.e. a large air burst). The acoustic pressure wave and its reflection off the ocean floor are detected by the sensor arrays. The data from the sensors is then processed and analyzed to determine optimum drilling locations or to monitor the status of known reservoirs.
As the number and complexity of the acoustic sensors in these and related applications have increased, the associated signal processing electronics have likewise increased. Prior art systems use analog circuitry to interrogate the sensors, but these analog systems are subject to drift and are very difficult to accurately calibrate. Also, recent developments have provided fiber optic interferometric sensors which provide unique signal processing problems (see "Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier" by Anthony Dandridge, Alan B. Tveten, and Thomas G. Giallorenzi, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, herein incorporated by reference). Note that this reference teaches a modulation/demodulation technique in which the I and Q frequencies are different. This technique is employed in the present invention.
Thus, there is a need for a signal processing system which overcomes the shortcomings of the analog systems and can be used with fiber optic interferometic sensor arrays. Specifically, there is a need for an improved calibration system for use with a digital demodulation system.